This invention relates generally to inkjet printing and more particularly to inkjet printheads.
Inkjet printing technology is used in many commercial products such as computer printers, graphics plotters, copiers, and facsimile machines. Generally, inkjet printing employs a printhead that ejects drops of ink through a plurality of nozzles or orifices onto a print medium such as a sheet of paper. One common printhead architecture used for thermal inkjet printing comprises a substrate having at least one ink feed hole and a plurality of ink drop generators arranged around the ink feed hole. Each ink drop generator includes a firing chamber in fluid communication with the ink feed hole. A heating element such as a resistor is located in each firing chamber. Ink is caused to be ejected through a selected nozzle by passing current through the associated resistor, which heats the ink in the firing chamber to a cavitation point. The resistors are typically formed as part of one or more thin film stacks disposed on top of the substrate. It is common to stagger the resistors relative to one another (a feature known as “resister stagger”) to improve performance of the printhead.
Printheads are commonly fabricated on a silicon wafer substrate using photolithography techniques. With this approach, it is possible for the thin film stacks to become undercut during etching of the ink feed hole. Thin film undercut generally varies between 8-10 microns, with occasional excursions up to 12-14 microns. This undercut presents a relatively fragile area that can fracture under stress experienced during operation.
Another issue with the above-described printhead architecture pertains to shelf length. As used herein, the term “shelf length” refers to the distance, for a given ink drop generator, from the center of the resistor to the edge of the ink feed hole adjacent to the ink drop generator. Here, the shelf lengths are relatively long (30-45 microns) and unequal because of resistor stagger. This results in increased nozzle-to-nozzle drop weight variability and reduced refill rates, which leads to less uniform printing and lower frequency operation.